Twenty-one adult Macaca mulatta that received 6000 rads of supervoltage X-irradiation in 30 fractions, alternate sides of the brain being exposed on alternate days, showed widely scattered minute necrotic lesions in the forebrain white matter in one out of three monkeys at 33 weeks, two out of three at 52 weeks, and two out of three at 104 weeks. With passage of time there was an increasing number of lesions with a trend toward mineralization except for those that extended into the cerebral cortex. Accompanying the necrotic lesions were both focal vascular endothelial hyperplasia and patches of telangiectasis. This project has been completed and the results published.